A conventional low pressure casting machine comprises a holding furnace having a holding chamber substantially filled with a pool of molten metal and a vented mold or other molten metal-receiving member mounted on top of a pressure-tight furnace lid or cover. The mold or other molten metal-receiving member is mounted on a fixture that is in communication with a riser tube that extends through the furnace lid and into the pool of molten metal. A gas under pressure is introduced into the holding furnace chamber above the molten metal pool whereupon the molten metal flows upwardly through the riser tube into the mold. Such machines are called "low pressure" casting machines because the pressure exerted on top of the metal pool within the holding furnace is only on the order of three to ten pounds per square inch above atmosphere.
Low pressure casting processes are essentially non-turbulent. Since molten aluminum which has been agitated, particularly in air, is less dense and of lower quality because of higher levels of oxide inclusions than metal which has not been agitated, parts produced by low pressure casting processes are often denser and of higher quality than parts produced by other casting operations.
Although there is minimal agitation of the aluminum during a low pressure casting operation, a problem is encountered with many known low pressure aluminum casting operations because there is no satisfactory way to deliver molten metal to the low pressure holding furnace which does not cause the molten metal to be exposed to air and agitated during the delivery process. To fill a low pressure holding furnace with molten metal, molten metal which has been transferred out of a metal melting furnace (or a holding furnace located adjacent the low pressure casting machine) is poured into the low pressure holding furnace by a transfer device, such as a ladle. (It will be understood that a melting furnace also serves as a holding vessel.) To do this, it is usually first necessary to open a pressure-tight cover over the holding furnace, transfer the molten metal into the holding furnace, and replace the pressure-tight cover. During these operations, the metal is agitated by the transferring and pouring operations so that molten metal in the holding furnace may already be significantly agitated before the casting operations are begun. These are time-consuming and expensive operations which may produce parts having less than the desired quality.
Commonly-owned U.S. Pat. No. 5,590,681, which is incorporated by reference above, discloses a low pressure casting process and apparatus wherein molten aluminum is delivered to a low pressure holding furnace through a launder system connected to a conventional melting furnace (or connected to a separate molten metal holding furnace). A valve assembly is provided in the launder system to selectively deliver molten aluminum from the melting furnace (or separate holding furnace) to the low pressure holding furnace. Typically, such valve assembly includes a rotatable valve element having a longitudinal passageway therethrough which may be aligned with passageways in the launder system leading to the low pressure holding furnace. To seal the low pressure holding furnace during a casting cycle, the valve element is rotated to move the passageway through the valve element out of alignment with the passageway leading to the low pressure holding furnace.
As explained in the '681 patent, the valve element is preferably formed from graphite, which is non-wetting and provides excellent resistance to molten aluminum. However, graphite is highly susceptible to oxidization in the high-temperature environment of metal casting. In addition because the rotatable valve element frictionally bears against a valve body during rotation of the valve element, a concern arises regarding deterioration of the valve element, due to friction, if the valve element is formed from graphite. As a result of such deterioration, molten metal may be undesirably forced, i.e. leak, upwardly around the valve element due to the pressure applied to the pool of molten metal in the low pressure holding furnace. Because such leakage of molten metal around the valve element occurs above the level of the molten metal in the valve assembly, the leakage not only presents a potential hazard to persons working around the valve assembly, but may also solidify and cause the valve element to freeze up.
An alternative valve construction disclosed in the '681 patent utilizes a vertically-movable, plunger-style valve element that closes a valve port centered about a vertical axis. The use of a plunger-style valve element is preferable to the aforementioned rotatable valve element in instances where molten aluminum is to be delivered to the low pressure holding furnace at relatively high rates because rotatable valve elements perform best with relatively small valve ports, whereas plunger-style valve elements perform satisfactorily with relatively large valve ports. However, the plunger-style valve disclosed in the '681 patent has certain drawbacks because mating parts of the valve element and the valve port are formed from non-wetting ceramic material. Consequently, the resulting valve seal is formed by the engagement of two hard surfaces, thereby requiring precision manufacturing to obtain a repeatable valve seal.